Active transport of anionic and cationic drugs plays a critical role in their intestinal absorption, penetration across the blood-CSF and blood-brain barriers, and accumulation within excretory organs. We have used oocytes and cell lines expressing rat and human OAT1 to examine their affinity for the nucleoside phosphonate antiviral drugs, adefovir and cidofovir. Both were are excellent substrates for these renal drug transporters, with the human isoform demonstrating particularly high affinity (Km = 4 uM). This observation explains the extensive renal accumulation these drugs and the associated human nephrotoxicity. We have also shown that OAT1 is present at the apical (CSF-facing) membrane of the choroid plexus, suggesting that OAT1 mediated CSF to blood transport may contribute to the limited penetration of these drugs in the brain and thus, to their limited effectiveness in combating CNS viral infection. However, recent data indicates that, despite the presence of apical OAT1 in choroid plexus, the nucleotide phosphonates are not well transported from CSF into the CP. Since other OAT1 substrates, e.g., the herbicide 2,4-dichlorophenoxy acteic acid, are very well transported by choroid plexus, this was a surprising finding. Recent studies of another organic anion transporter, provide the explanation. When expressed in oocytes, OAT3 does not transport the nucleoside phosphonates. Furthermore, the choroid plexus of OAT3 knockout mice do not transport a number of OA, indicating that quantitatively it is the major OAT there. In collaboration with Dr. David Miller, we have also used confocal microscopy to examine the cell to blood step in choroid plesus drug and xenobiotic transport. It appears that the ATP-driven drug pump, MRP2, is found at the apical (facing the vascular lumen) membrane of the blood-brain barrier and the apical (CSF facing) membrane of choroid plexus. The basolateral (blood-side) transporter in choroid plexus appears to be MRP1. We have also shown that the nucleoside phosphonates are substrates for ATP-driven MRP2-mediated transport at other sites, and preliminary data indicates that the MRPs do play a role in the barrier function of choroid plexus for the nucleoside phosphonates.